Filter arrangement applicable to ADSL splitters

ABSTRACT

A low-pass filter arrangement associated a clamping circuit for limiting unwanted transient signals to be transmitted through the filter. Mainly the variations (dI/dt) of the current flowing through the filter are eliminated. The filter may form part of a splitter used in an ADSL system to separate low frequency POTS signals from high frequency ADSL signals simultaneously transmitted over a same copper twisted-pair transmission line. The filter comprises the series connection of a coil and the main path of a transistor of which the gate terminal is connected to the opposite end of the coil. In an ADSL system, a classical 7th order low-pass filter may advantageously be replaced by a 3rd order filter arrangement.

BACKGROUND OF THE INVENTION

The present invention relates to a filter arrangement to filter signalstransmitted between a first filter port and a second filter port, saidarrangement including filter means adapted to filter predeterminedfrequency components of said signals, and associated clamping meansadapted to limit the transmission of unwanted signals between said firstfilter port and said second filter port.

Such a filter arrangement is generally known in the art and is used toisolate the first filter port from the second filter port, and therebythe circuits, devices or apparatus connected to these ports. The signalsmay for instance be provided by a power supply or may betelecommunication signals whereby the filter arrangement is then used toseparate different signals transmitted on a same telecommunication linebut intended to different apparatus, as it is the case in a “party line” system where two telephone sets share a same transmission line, e.g.connected to the second filter port. Also many telecommunicationmodulation schemes use Frequency Division Multiplexing (FDM). A filterarrangement is then needed to filter the signals that an apparatus,connected to the first filter port, needs from the signals it doesn'tneed. Another important application of such a filter arrangement may befound in the Asymmetric Digital Subscriber Line (ADSL) systems, as wellas in systems derived therefrom such as HDSL, VADSL, etc. As well known,these systems provide broadband services over a transmission lineconstituted by a classical twisted pair of copper wires, whereby a usermay use these services without the need to wait for having a fiberconnected to its premises. Since the telephone signals of the so-calledPlain Old Telephone Service (POTS) are also transmitted on the sametelecommunication line, a “splitter” is necessary to separate the highfrequency ADSL signals, e.g. intended for an ADSL modem, from the lowerfrequency telephone signals, intended for a local telephone set. Such asplitter obviously includes a line filter arrangement. A high-passfilter is sitting in front of each modem, and a low-pass filter issitting in front of each POTS interface. Additionally to this frequencyfiltering, the line filter arrangement needs further also to be adaptedfor isolating the apparatus connected a local port, e.g. the firstfilter port, from the transmission line connected a line port, e.g. thesecond filter port, and this during severe changes in voltage/ currentfrom one of the sides of the arrangement. This may for instance be thecase of spikes on the signals and that have to be eliminated.

A splitter including a filter arrangement is for instance described inthe article “ADSL and VADSL Splitter Design and Telephony Performance”by J. Cook et al, published in the IEEE JOURNAL ON SELECTED AREAS INCOMMUNICATIONS, VOL. 13, NO. 9, DECEMBER 1995, pages 1634 to 1642.Therein is mentioned that splitters for ADSL, and more particularly thefilters included therein, are preferably based on passive filters. Themain reason therefore is that they obviate the power provision issuesand ensure continued functioning in the event of power failure. Passivefilters are also able to cope more easily with large signals, bothvoltage and current, inherent in telephony signaling and powering.Passive filters have a linear behavior as they mostly use inductors andcapacitors, whilst active filters use operational amplifiers andresistors. Such a linear approach makes the design relatively easybecause the Laplace-transforms and network technology can be used. Hereagain, there is a lot of literature available on these topics. However,this easy design is only valid if everything remains linear in allconditions. In practice however, coils that saturate are no longerlinear. This means that big, and thus relatively expensive, coils needto be provided to take care that they do not saturate.

Another problem relating to ADSL applications is to avoid transientsignals to be transmitted from the POTS to the ADSL side, and viceversa. Transient signals are disturbances produced, e.g., by polarityreversal of the battery or by starting of the ringing signal at the sideof the telecommunication exchange, and/or by on-/ off-hook signals atthe (user) side of the telephone set. To protect against the negativeeffects of the transient signals, large and expensive high (e.g. 7^(th))order filters are required as low-pass filters. Such transient and otherover-current or over-voltage signals could, although not mentioned inthe above known prior art, also be absorbed by clamping means such asZener diodes adapted to eliminate voltage and/or current spikes.However, a problem with these known clamping means is that they operatein an abrupt way.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a filter arrangement ofthe above known type but which is adapted to filter transient signals ina more efficient way while being simpler and cheaper than the one knownfrom the prior art.

According to the invention, this object is achieved due to the fact thatsaid clamping means include active device means adapted to limitvariations of predetermined parameters of said signals.

In this way, the disturbances caused by transient or unwanted signalsare progressively, and no longer abruptly, annihilated by the clampingmeans. Tests have proved that the worst transient signals are bestattenuated by clamping means acting on the current variation or on thevoltage variation of the signals rather than on the absolute value ofthese parameters. A combined limitation on the current/ voltagevariations is also efficient.

In more detail, the present invention is characterized in that saidfilter means includes at least one reactive device having a first endconnected to said second filter port and a second end coupled to saidfirst filter port via a main path of said active device means whichfurther has a control terminal coupled to said first end of saidreactive device.

In this way, no power supply is required for the active device.

The present invention is further characterized in that said arrangementis a low-pass filter adapted to filter high frequency components ofsignals transmitted between said apparatus and said transmission line,in that said reactive device is an inductor, and in that said activedevice is a transistor.

It can be proved that, in case of ADSL signaling, the requirements onthe filter means to filter-out the high frequency components of thesignals are, in case the present arrangement is used, much less severethan those of the prior art: a 2 ^(nd)(or at most a 3 ^(rd)) orderfilter may now be used instead of the above mentioned 7 ^(th) orderfilter for obtaining the same results. Moreover, the inductor used inthe present 2 ^(nd) order filter may be smaller than that (or those) ofthe prior art because it will saturate less owing to the limitedcurrent/ voltage of the signals.

In a preferred embodiment, the present invention is furthercharacterized in that said arrangement is a low-pass filter adapted tofilter high frequency components of signals transmitted between saidfirst filter port, which has a first and a second local terminal towhich said apparatus is connected, and said second filter port, whichhas a first and a second line terminal to which said transmission line,that is a double-wired line, is connected, in that said filter meanscomprises a first inductor and a second inductor as reactive device andthat said active device means comprise a first transistor and a secondtransistor, in that said first line terminal is connected to the firstend of said first inductor, the second end of said first inductor beingcoupled to said first local terminal via the main path of said firsttransistor, and the control terminal of said first transistor beingcoupled to said first end of said first inductor, and in that saidsecond line terminal is connected to the first end of said secondinductor, the second end of said second inductor being coupled to saidsecond local terminal via the main path of said second transistor, andthe control terminal of said second transistor being coupled to saidfirst end of said second inductor.

In this way, the arrangement is adapted to limit either positive andnegative spikes of the transmitted signals, and it operates in a fullybi-directional way.

Another characterized feature of the present invention is that saidactive device means further comprise a third transistor and a fourthtransistor, that said first line terminal is connected to the first endof said first inductor via the main path of said third transistor, thecontrol terminal of said third transistor being coupled to said secondend of said first inductor, and that said second line terminal isconnected to the first end of said second inductor via the main path ofsaid fourth transistor, the control terminal of said fourth transistorbeing coupled to said second end of said second inductor.

The present arrangement then shows balanced characteristics with respectto the devices connected thereto.

The present invention is further also characterized in that said firstand second line terminals are coupled via the series connected mainpaths of a pair of transistors of which the control terminals areconnected to the first ends of said first and second inductorsrespectively.

The currents limited by the filter means and their associated clampingmeans are then derived through this pair of transistors so that thepresent filter arrangement remains transparent for the device connectedthereto.

Further characteristic features of the present filter arrangement arementioned in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will becomemore apparent and the invention itself will be best understood byreferring to the following description of an embodiment taken inconjunction with the accompanying drawings wherein:

FIG. 1 is an overview of an ADSL system including a filter arrangementLP1, LP2 according to the invention,

FIG. 2 shows the basic circuit LP of a filter arrangement LP1, LP2 ofFIG. 1, and

FIG. 3 is a practical implementation LP of the filter arrangement LP′ ofFIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Although not limited to any particular field of use, the filterarrangement of the present invention is particularly applicable to asplitter for an Asymmetric Digital Subscriber Line “ADSL” system asshown in FIG. 1. As known, an ADSL system—or a similar system such asHDSL, VADSL, . . . —is adapted to transmit voice and higher frequencysignals over a classical copper twisted-pair telephone line TP. At eachend of the twisted-pair transmission line TP is provided a splifter thatseparates the high frequency ADSL signals from the low frequency PlainOld Telephone Service POTS (or voice) signals. In more detail, at thecustomer premises side CP a splitter SP1 has a line port LI1 thatconnects an end of the transmission line TP both to a port LM1 of alocal ADSL modem ADSLcp and to a local port LO1 of a user subscriber setPOTS, whilst at the central office side CO another splitter SP2 has aline port L12 that connects the other end of the transmission line TPboth to a port LM2 of a central ADSL modem ADSLco and to a local portLO2 of a Public Switching Telecommunication Network PSTN. To this end,the splitter SP1 includes a high-pass filter HP1 that interconnects theports LI1 and LM1, and a low-pass filter LP1 that interconnects theports LI1 and LO1. Similarly, the splitter SP2 includes a high-passfilter HP2 that interconnects the ports LI2 and LM2, and a low-passfilter LP2 that interconnects the ports LI2 and LO2.

The present invention concerns an improved version of the low-passfilter LP1 or LP2, hereafter more generally called LP as shown at FIG.2. This line filter arrangement LP is located between a first filterport LO and a second filter port LI, which respectively correspond tothe above local ports LO1/ LO2 and line ports LI1/ LI2. The purpose ofthe filter LP is to isolate its two sides during severe changes involtage/ current at one of these sides. As will be explained below, thisis done by limiting or clamping the current variation dI/dt, where I isthe current flowing through the filter. Although not described here,limiting the absolute value of the current I, or limiting the voltagevariation dV/dt as well as combinations of all these parameters is alsopossible. However, practical tests show that the latter provide slightlyless efficient results.

The port LI of the filter LP has two line terminals LIa and LIb, whilstthe port LO has two local terminals LOa and LOb. The line terminal LIais connected to a first end of an inductor or coil L1 of which thesecond end is coupled to the local terminal LOa via the source-to-drains-d (or main) path of a first FET transistor T1. The line terminal LIa,and thus the first end of the coil L1, is further also connected to thegate g or control terminal of the transistor T1.

As well known, the voltage over the coil L1 is proportional to thevariation dI/dt of the current I flowing therethrough. This voltageappears as gate-source voltage Vgs across the transistor T1 and thusdetermines what resistance the transistor T1 shows between its drain dand source s.

Under normal operation conditions, dI/dt is small. The induced Vgs istherefore also small and, since the transistor T1 is of the depletiontype, the resistance of its main path, i.e. between d and s, is small(about 3 Ohm) and will not change significantly. The transistor T1 canthen just be seen as an inter-connection and the filter operates as anormal classical inductance filter.

If the voltage, e.g. at the filter port LI, rises, the current I willincrease. If this voltage change is relatively fast, the Vgs willincrease rapidly and the resistance between d and s will also rise. Thisincrease of the main path resistance of the transistor T1 has the effectof decreasing the current I, thereby counteracting the rapid change inI. The result is that this feedback mechanism limits immediately thevariation dI/dt of the current I to a predetermined value. Owing to thiscontrol on the variation dI/dt, the current I will however reach itssame final value but much slower than without the FET transistor T1.

The above described filter arrangement LP including the inductor L1 andthe transistor T1 between the filter ports LI and LO is sufficient tolimit or clamp current variations in the one direction between theseports. However, in order to limit current variations in the twodirections and for both possible positive and negative spikes of thesignals flowing through the filter, the series connection of a secondcoil L2 and second transistor T2, arranged in a same way andrespectively similar to L1 and T1, needs to be provided between the lineterminal LIb and the local terminal LOb. Preferably, and in order toensure the symmetry of the arrangement, the coils LI and L2 are thenmagnetically coupled as shown in FIG. 2.

The FIG. 3 shows a more practical implementation LP′ of the filterarrangement LP. Additionally to the components mentioned above inrelation with LP, the filter LP′ includes a third transistor T3 of whichthe main path connects the line terminal LIa to the first end of thecoil L1, and a fourth transistor T4 of which the main path connects theline terminal LIb to the first end of the coil L2. The arrangement LP′then has balanced characteristics with respect to the devices connectedthereto and the negative effect of possible common-mode signals are soeliminated To better control the operation of the filter, the gate orcontrol terminals of the transistors T1, T2, T3 and T4 are connected tothe opposite end of their associated coil via a resistor R1, R2, R3 andR4 respectively.

In case of an ADSL system as shown in FIG. 1, a 7 ^(th) order low-passfilter is generally needed to filter-out the transients of the signals.It has however been proved that by limiting the variation dI/dt of thecurrent I as by the present arrangement, the filter may be reduced to a3_(rd) order filter while providing the same results. Such a 3 ^(rd)order filter is obtained by inserting a capacitor C1 between the lineterminals LIa and LIb and a second capacitor C2 between the localterminals LOa and LOb. A 2 ^(nd) order filter should also be acceptablebut then with less efficient results.

The filter LP′ is further protected against over-current andover-voltage by means of Zener diodes Z1, Z2, Z3 and Z4 connectedbetween the gate and the source electrodes of the transistors T1, T2, T3and T4.

Finally, a pair of series connected transistors T5 and T6 is providedbetween the local terminals LOa and LOb. The gate or control terminal ofT5 is connected to the gate of T1, whilst the gate of T6 is connected tothat of T2. Contrarily to the transistors T1 to T4, that all are of thedepletion type, the transistors T5 and T6 are of the enhanced type.These transistors operate in a complementary way with respect to thetransistors T1 to T4. In other words, when the current flow through thetransistors T1-T4 decreases, the current flow through the transistorsT5-T6 increases. It can be seen as the currents limited by T1-T4 arederived towards T5-T6, and that the current clamping operation istransparent for the devices connected to the present filter arrangement.

Although the above description has been made with respect to a low-passfilter arrangement, a high-pass filter arrangement may also be designedon the same principle, mainly by replacing the inductors by capacitors.

While the principles of the invention have been described above inconnection with specific devices, it is to be clearly understood thatthis description is made only by way of example and not as a limitationon the scope of the invention, as defined in the appended claims.

What is claimed is:
 1. Filter arrangement to filter signals transmittedbetween a first filter port and a second filter port, said arrangementcomprising: filter means to filter predetermined frequency components ofsaid signals, and clamping means to limit the transmission of unwantedsignals between said first filter port and said second filter port,wherein said clamping means comprise active transistor means to limitvariations $\frac{I}{t},\quad \frac{V}{t}$

 of predetermined parameters of said signals, wherein said arrangementis a line filter arrangement of which said first filter port is a localport connected to an apparatus and said second filter port is a lineport connected to a transmission line and said arrangement is a low-passfilter to filter high frequency components of signals transmittedbetween said apparatus and said transmission line, and said filter meanscomprises a reactive device that is an inductor.
 2. Filter arrangementaccording to claim 1, wherein said reactive device has a first endconnected to said second filter port and a second end coupled to saidfirst filter port via a main path of said active transistor means, andsaid active transistor means has a control terminal coupled to saidfirst end of said reactive device.
 3. Filter arrangement according toclaim 1, wherein said apparatus is connected to a first and a secondlocal terminal of said local port, in that said transmission line is adouble-wired line connected to a first and a second line terminal ofsaid line port, and in that said first line terminal is connected to thefirst end of said inductor, the second end of said inductor beingcoupled to said first local terminal via the main path of saidtransistor, and the control terminal of said transistor being coupled tosaid first end of said inductor.
 4. Filter arrangement according toclaim 1, wherein said arrangement has a first and a second localterminal to which said apparatus is connected, and said second filterport, which has a first and a second line terminal to which saidtransmission line, that is a double-wired line, is connected, in, thatsaid filter means comprises a first inductor and a second inductor asreactive device and that said active transistor means comprise a firsttransistor and a second transistor, in that said first line terminal isconnected to the first end of said first inductor, the second end ofsaid first inductor being coupled to said first local terminal via themain path of said first transistor, and the control terminal of saidfirst transistor being coupled to said first end of said first inductor,and in that said second line terminal is connected to the first end ofsaid second inductor, the second end of said second inductor beingcoupled to said second local terminal via the main path of said secondtransistor, and the control terminal of said second transistor beingcoupled to said first end of said second inductor.
 5. Filter arrangementaccording to claim 4, wherein said first and said second inductors aremagnetically coupled.
 6. Filter arrangement according to claim 4,wherein said active transistor means further comprise a third transistorand a fourth transistor, in that said first line terminal is connectedto the first end of said first inductor via the main path of said thirdtransistor, the control terminal of said third transistor being coupledto said second end of said first inductor, and in that said second lineterminal is connected to the first end of said second inductor via themain path of said fourth transistor, the control terminal of said fourthtransistor being coupled to said second end of said second inductor. 7.Filter arrangement according to claim 4, wherein said first and secondline terminals are coupled via the series connected main paths of a pairof transistors of which the control terminals are connected to the firstends of said first and second inductors respectively.
 8. Filterarrangement according to claim 4, wherein said clamping means furthercomprises over-voltage protection means coupled across said first andsecond inductors respectively.
 9. Filter according to claim 1, whereinthe signals transmitted on said transmission line are of an AsymmetricDigital Subscriber Line type.
 10. Filter according to claim 4, whereinsaid first and said second local terminals are shunted by a firstcapacitor, and in that said first and said second line terminals areshunted by a second capacitor.
 11. Filter arrangement according to claim1, wherein said active transistor means comprise field effecttransistors.